The installation of equipment for subsea fluid extraction wells involves the lowering of heavy assemblies onto the sea bed. It is particularly difficult to lower components such as subsea control modules to locate on structures already on the sea bed, such as a well tree, as considerable positional accuracy is required. The lowering of such components is normally effected from a surface vessel, in conjunction with the use of a subsea remote operated vehicle (ROV). However, the surface vessel is subjected to the conditions of the surface sea state, causing the vessel to move in pitch, yaw and heave. The effects of pitch and heave are minimised by the use of vessels which are purpose designed to allow lowering from a special access in the center of the vessel. However since the availability of such vessels is limited, their use is expensive. It is therefore desirable to use a “vessel of opportunity” i.e. one which is not purpose designed for this work, in conjunction with the ROV, thus increasing the availability of vessels suitable for installation and so substantially reducing costs for the operator.